boost phase (thing)

That initial part of the flight of a missile or spacecraft when its rocket motors are firing. For spacecraft, this usually refers to the launch; motor firings after achieving space are usually called burns. This phrase has become more frequently heard recently with the debate in the U.S. over missile defense; one type of missile defense system (the much, much more feasible kind) involves hitting enemy missiles during this phase, just after launch; this is known as a boost phase intercept.